According to an inkjet recording method, high speed recording can be performed with a high freedom degree of imaging pattern and a low noise at the time of recording. Further, image recording can be performed at low cost. Still further, the inkjet recording method has advantages such that color recording can be readily performed. Therefore, recently the inkjet recording method is rapidly spreading and further developing. As an inkjet recording ink for the method, hitherto a dye ink, in which a water-soluble dye is dissolved in an aqueous medium, has been widely used. However, the dye ink is poor in water resistance and weather resistance of the resultant printed article. Therefore, a pigment ink, which has a potential to improve the above problems, has been studied.
However, pigments are often inferior to dyes in terms of discharge (emission) property from a nozzle of inkjet printer head. Besides, pigments are not in a single molecule state like dyes, but in a particle state. Accordingly, owing to scattering and reflection caused by pigments, absorption spectra of pigments are ordinarily broader than those of dyes. As a result, there is a tendency that the image formed from a pigment ink generally has lower transparency and lower color formation efficiency than those of the image formed from a dye ink. A means for resolving these problems is to reduce the size of particles. It has been desired to make the pigment into fine particles so that light scattering would be reduced and the particles would show as good transparency as dyes. Formation of fine particles of pigment is ordinarily performed by a mechanical force using a dispersion machine such as a sand mill, a roll mill, and a ball mill. However, the method of using such a mechanical force can pulverize pigment into a size of about 100 nanometers at the best, which is a vicinity of a primary particle size. Therefore, this method can hardly cope with a demand for further size reduction of particles (JP-A-10-110111 (“JP-A” means unexamined published Japanese patent application)). Further, as a desired size of particles becomes finer, not only it takes longer time to disperse them and requires greater costs but also it becomes more difficult to obtain particles with a uniform quality.
Recently, some other methods are proposed, which includes a method of preparing fine particles of pigments by once dissolving the particles in a solvent to form a solution and then forming (depositing) fine particles using the solution, and a method of concentrating organic particles contained therein (JP-A-2004-43776, JP-A-2006-342316, and JP-A-2007-119586). The thus-obtained dispersion of organic pigment particles shows a reduced light scattering and a higher transparency than the dispersion prepared by an ordinary pulverizing method. However, the thus-obtained fine particles having a particle diameter in nanometer order still have problems: it is so difficult to impart stability to the dispersion that fine particles aggregate with each other in the lapse of time, and resultantly viscosity increases. Further, the problem that light fastness deteriorates by use of fine particles has not been solved yet.